This invention relates to the field of digital data communication and more particularly to a system or apparatus for rendering such communications resistant to EMI (electromagnetic interference), for example under conditions experienced in high performance military aircraft.
Old methods of providing reliable communications include (1) using a high power (relative to expected interference) transmitter, (2) using filters to pass only those frequencies that are used in the particular instance, (3) using various modulation techniques to transfer the information via certain characteristic(s) of the transmitted signal (e.g. amplitude, frequency, phase), (4) using various codes to allow detecting and correcting errors that occur in the communication process. Generally, several of the above methods are used.
The principal disadvantage of the old methods is the inability to adapt to the specific interference that happens to be present as each portion of a message is transfered from its source to its destination. The old methods deal (at best) with the characteristics of the interference on a statistical basis, rather than with the characteristics of the specific interfering signal. This disadvantage is due to the following two limitations: (a) transfering information at a fixed rate (i.e. independent of the effects of interference on the current signal), and (b) transmitting the same signals each time the same message is sent (assuming the same initial state of the transmitter).
Transmissions of data among a plurality of avionics subsystems on the same aircraft have been subject to interruption by electromagnetic interference, even when transmission is by shielded wire.